Condenser servo mechanism



June 2, 1953 Q w, s wm 2,640,954

CONDENSER SERVO MECHANISM Filed Jan. 15, 1946 g" FIGJ W POWER AMPLIFIER 6+ AND ANTI -HUNT MOTOR CIRCUITS HIGH FREQUENSIY SOURC;

(LII

POWER AMPLIFIER FREQUENCY 8+ AND ANTI-HUNT MOTOR v SOURCE CIREUITS INVENTOR.

CHALMERS w. SHERWIN BY w J I ATTORNEY Patented June 2, 1953 UNITED STATES PATENT OFFICE by mesne assignments, to the United States" of' America as represented by the Secretary of the Navy Application January 15, 1946,, SerialNm. 641,347.

Claims. 1

This invention relates to servo systems" and more particularly to servo systems usin variable impedance elements.

Heretofore servo systems, such as those using synchrons, had the disadvantage that they were bulky, of considerable weight, and required an appreciable torque at the sending end.

A general object of the present invention is to overcome the foregoing difiiculties.

Another object of the present invention is to provide a servo system using variable impedance elements.

A further object is to provide a servo system which is light and compact.

A still further object is to provide a servo system which requires verylittle torque at its sending end.

These and other objects will be apparent from the following specification when taken with the accompanying drawing, in which:

Fig. 1 is a schematic illustration of one form of the invention;

Fig; 2 is a schematic illustration of another form of the invention; and

Fig. 3 is a plan view of the variable condensers.

Referring to the drawings, and more particularly Fig... 1, a servo system is shown having two variable impedance elements, and more particularly a variable condenser H at the transmitting end of the system and another variable condenser t9 atv the receiving end of the system. A comparatively" high frequency voltage source it is modulated by a comparatively low frequency voltage source. applied at terminal l-l-. This modulated voltage at 12 is applied to similar voltage divider networks comprising condensers l6 and. H and I8 and [9, respectively such that variable condensers l1 and [9 have fractions of the modulated voltage across them. Peak detector circuits comprising electron. tubes 35 and 4.6 and their other circuit components detect the modulation envelopes of the fractional voltages on condensers l1 and. [.9 respectively. A difference amplifier comprising, electron tubes 3! and M, a. common cathode coupling resistor 51, and an. output transformer 2|, compares the amplitudes of the aforesaid. modulation envelopes. The output voltage from transformer 2|, being at the same frequency as low frequency voltage applied at H, is applied to a power amplifier l3 incorporating conventional anti-hunt circuits and the output therefrom is applied to motor I l. The combination of the. output from power amplifier l3 and a voltage from a source at terminal. ll causes motor 14 to rotate and drive the rotor of variable condenser I9 so that its capacitance is essentially that of condenser ll.

Referring to Fig. 2 resistor 5-! (Fig. 1) may comprise an electron tube 52 and its associated circuit component such that the cathode cou- 2. pling circuit of the difference amplifier has high dynamic resistance and low static resistance. This provides greater output from. the differ.- ence amplifier while permitting. tubes 3! and 41 to drawsuflicient current to operate in their high gain region.

Referring to Fig; 1', a high frequency or R.-F. voltage source 12 is modulated. by a low frequency voltage source applied: at terminal II. This low frequency voltage is also applied to one phase ('not shown) of a two phase induction motor M. Inone embodiment of this invention cycle voltage was used for this low frequency voltage source.

The modulated high frequency voltage from source 12 is applied to two similar voltage divider networks comprising condensers l5 and il and condensers l8 and t9", respectively, wherein con.- densers ll and [9 are rotatable and variable. The use of modulated highfrequency voltage permits condensers I 6, I'I, t8 and I9 to be small"- er than if the low frequency voltage had been used directly. This feature of the invention is particularly advantageous if the transmitting end condenser IT is tobe drivenby atransmitting deviceable' to furnish only a very small amount of torque or linear motion.

Since this servo system merely compares the amplitude of the fractional modulated voltages across condensers I l and i9, high frequency detectors circuits are provided, as shown in Fig. 1, to recover the modulation envelopes these voltages; This minimizes any" errors due to phase-shifts and also converts back to a frequency suitable for use on the other phase (not shown) of motor M. The above mentioned high fre quency detector circuits comprise, for the signal across; condenser I1, and electron tube 36, resistors 24' and. 26', and condenser 25, and for the signal across condenser [9, an electron tube 46-, resistors 29: and" 215, andv condenser 28. In one of these detector: circuits electron tube 36 has its plate. 321: connected to a source of positive potenti'al at its. grid. 38. connected to a grid resistor 24 and the junction of condenser l6 and ll, and its; cathode 39 connected to a high frequency filter circuit comprisingresistcr 2t and condenser Z5 in parallel. The output fromthis detector is obtained at cathode 39 and comprises essentially only the modulation envelope of the voltage acrosscond'enser H. The other detector circuit is similarly connected and providesv at cathode 45 of tube 46 an output voltage comprising essentially only the modulation envelope. of. the voltage across, condenser 19;.

The outputvoltages of tubes 3.6 and 46 are applied to. grids 33. and. lsrespectively of the differ.- ence amplifier. The electron. tubes 3i and 4|- comprisin this, difference.- amplifier. have. their.

cathodes 34 and M respectively connected together and to a common cathod coupling resistance comprising either resistor (Fig. 1) or electron tube 52 and its associated circuit components (Fig. 2). The plates 32 and 42 of tubes 31 and 4! respectively are connected to opposite terminals of primary winding 23 of transformer 21, the centertap of primary Winding 23 being connected to a positive source of potential at 15. The output voltag from the difference amplifier is obtained from the terminals of secondary winding 22 of transformer 2i, and is proportional to the difference of the voltages across condensers I! and i9.

This output voltage is applied to power amplifier l3 which incorporates conventional antihunt circuits and the output therefrom is applied to the other phase of induction motor [4. If there is a difference in the voltages applied to rids 33 and 43 the aforementioned output voltage causes motor M to turn so that its output shaft moves the rotor of condenser is at the receiving end of the system to make its capacitance essentially equal to that of condenser I! and thus decrease the difference in the voltage at grids 33 and 43. The above constitutes a servo or follow-up system in which the rotor of condenser l9 at the receiving end is positioned with respect to the rotor of condenser I! at the transmitting end.

If high angular accuracy is desired over a small angle a condenser such as that depicted in Fig. 3 may be used for the variable condensers I! and IS. The condenser of Fig. 3 has a toothed rotor 65 and a slotted stator 62 such that a small angular change of rotor 6| with respect to stator 62 will produce a large change of capacitance. Such design eliminates the necessity of any gearing at the transmitting end which would load down the transmitting device. Those skilled in the art will realize that many other condenser designs are possible and it is to b understood that this invention is not to be limited to the condenser shown in Fig. 3.

In order to facilitate initialalignment of condenser [9 with respect to condenser 11, condenser [6 or it or both may be made variable so that the difference of the voltages at grids 33 and 43 are zero when transmitter and receiver are aligned with respect to one another. Other methods for producing such alignment will suggest themselves to those skilled in the art.

It will now be apparent that this invention has the advantages of being light and compact and requiring a small amount of torque from the transmitting device. The use of modulated high frequency voltage reduces the size of the variable condensers used. The system is not sensitive to amplitude or frequency changes of the voltages at source ll or l2.

The invention described in the foregoing specification need not be limited to the details shown, which are considered to be illustrative of one form the invention may take.

What is claimed is:

1. In an electrical positional control system, a circuit comprising two variable voltage dividers each including a variable impedance element and a constant impedance element, a source of alternating voltage Of comparatively high frequency, a second source of alternating voltage of comparatively low frequency modulating said first source, means for applying said modulated voltage to said voltage dividers, first and second high frequency peak detectors for demodulating the voltages impressed on said variable impedance elements, a difference amplifier for comparing the amplitudes of the output voltages of said first and second detectors and means for positioning one of said variable impedance elements in response to said comparison.

2. In an electrical positional control system, a circuit comprising two variable voltage dividers each including a rotatable variable capacitor in series connection with fixed capacitor, a source of high frequency alternating voltage modulated by a low frequency alternating voltage, means for applying said modulated voltage to each of said voltage dividers, electron tubes for detecting the modulation envelopes of the voltage impressed across said variable capacitors, two additional electron tubes having a common cathode coupling resistance connected as a difference amplifier for comparing the amplitudes of the aforesaid modulation envelopes, and a motor driven by the output voltage of said difference amplifier, whereby one of said variable capacitors is angularly positioned such as to make the voltages appearing across said variable capacitors substantially equal.

3. The circuit of the electrical positioning control system of claim 2 wherein said common cathode coupling resistance comprises an electron tube having high dynamic plate resistance and low static resistance.

4. In an electrical positional control system, a circuit comprising two variable voltage dividers each including a variable capacitor and a fixed capacitor in series connection, a source of high frequency alternating voltage modulated by a low frequency alternating voltage, means for applying said modulated voltage to each of said voltage dividers, a high frequency demodulator responsive to the potentials impressed on each variable capacitor of said voltage dividers for producing a demodulated output potential, a difference amplifier responsive to said demodulated potentials to produce an output potential proportional to the difference Of said demodulated potentials, and means for positioning one of said variable capacitors in response to said output potential to make said output potential a minimum.

5. In an electrical positional control system, a circuit comprising two variable voltage dividers each including a variable capacitor in series connection with a fixed capacitor, a source of high frequency alternating voltage modulated by a low frequency voltage, means applying said modulated voltage to each of said voltage dividers, a high frequency peak detector for detecting the modulation envelope of the voltage impressed on each of said variable capacitors, a difference amplifier responsive to the respective amplitudes of said modulation envelopes for producing an output proportional to the difference of said amplitudes, and motor means driven from said output whereby one of said variable capacitors is angularly positioned to make said output a minimum.

CHALMERS W. SHERWIN.

References Cited in the file of this patent UNITED STATES PATENTS Number 

